Typical examples of liquid crystal cell include a field effect mode cell proposed by M. Schadt et al., Applied Physics Letters, 18, 127-128 (1971), a dynamic scattering mode cell proposed by G. H. Heilmeir et al., Proceeding of the I.E.E.E., 56, 1162-1171 (1968), and a guest/host mode cell proposed by G. H. Heilmeir et al., Applied Physics Letters, 13, 91 (1968) or D. L. White et al., Journal of Applied Physics, 45, 4718 (1978).
These liquid crystal display cells are required to satisfy various characteristics, with quick response and low threshold voltage (V.sub.th) being the most important ones. Liquid crystal display cells having a low threshold voltage can be driven at a low voltage. Quick response liquid crystal display cells, on the other hand, can be fabricated by using liquid crystal materials low in viscosity, since the response time (.tau.) is directly proportional to the viscosity (.eta.) of the liquid crystal material, i.e., .tau..varies..eta..
Among these liquid crystal display cells mentioned hereinbefore, the TN mode cells which belong to the field effect mode cells, are mainly used at present. In the case of the TN mode cells, it is required to set the product of the optical anisotropy (.DELTA.n) of the liquid crystal material in the cell and the thickness (d).mu.m of the cell to a definite value in order to achieve good cell appearance, as indicated by G. Bauer, Mol. Cryst. Liq. Cryst., 63, 45 (1981). A liquid crystal display cell used in practice has a .DELTA.n.multidot.d value selected from among 0.5, 1.0, 1.6, and 2.2. In general, the viewing properties of a liquid crystal display cell can be improved by setting the .DELTA.n.multidot.d value to 0.5. On the other hand, the frontal contrast thereof can be ameliorated by setting the .DELTA.n.multidot.d value to 1.0, 1.6, or 2.2. Therefore, it is generally recommended to set the .DELTA.n.multidot.d value of a liquid crystal display cell to 0.5 when it is considered more important to achieve excellent viewing properties from any direction. On the other hand, the .DELTA.n.multidot.d value thereof is preferably set to 1.0, 1.6, or 2.2 to obtain a clear frontal contrast.
However, the thickness in a liquid crystal display cell used in practice is commonly set to a definite value within a limited range of from 6 to 10 .mu.m. Thus, a liquid crystal material having a low .DELTA.n is required to set the .DELTA.n.multidot.d value to 0.5. In contrast thereto, a liquid crystal material having a high .DELTA.n value is requisite to obtain a .DELTA.n.multidot.d value of 1.0, 1.6, or 2.2. That is, either a liquid crystal material of a low .DELTA.n value or one of a high .DELTA.n value is required depending on the desired display properties.
In the TN mode cells, the dielectric anisotropy (.DELTA..epsilon.) of a mixed liquid crystal is required to be positive. Therefore, it is demanded to provide nematic liquid crystal materials which are large in .DELTA..epsilon. value and low in threshold voltage, hence capable of being driven at a low voltage.
As a viscosity depressant known to the present to use for the purposes mentioned hereinbefore, mentioned is a compound represented by formula (a) below ##STR3##
However, the compound represented by formula (a) above suffers a disadvantage, since the addition of this compound to a liquid crystal matrix commonly used at present as the nematic liquid crystal composition reduces the viscosity of the liquid crystal matrix at one hand, and increases the threshold voltage on the other.